Vehicles, such as motor vehicles, may include a start-stop system configured to shut down an engine of the motor vehicle automatically when the engine is not required, e.g., when torque from the engine is not required to drive the vehicle, and to restart the engine automatically when desirable. Shutting down the engine in such circumstances reduces the idling time of the engine, increases fuel efficiency, and reduces vehicle emissions. Such an engine shut down may be referred to as an idle-stop. In some examples, the start-stop system determines when to shut down and restart the engine according to a brake pressure or braking torque of the brake system. For example, when the brake pressure exceeds a threshold brake pressure value, the start-stop system may shut down the engine, and when the brake pressure drops below the threshold brake pressure value, the start-stop system may restart the engine. Additionally, conditions for the start-stop system shutting down the engine may include the vehicle traveling below a threshold speed, such as when the vehicle is stopped.
Drivers are typically able to judge a level (e.g., amount) of brake pressure for bringing a vehicle to a stop when the vehicle is traveling on a flat surface. After a period of driving a vehicle equipped with a start-stop system, the driver may develop a sense of the level of brake pressure for stopping the vehicle and a level of brake pressure that will lead to the engine being shut down when the vehicle comes to a stop. The driver is therefore able to modulate the brake pressure to achieve a desired performance of the vehicle.
When the vehicle stops on an incline, an amount of braking torque (and therefore brake pressure) for holding the vehicle stationary is greater than when the vehicle stops on a flat surface. Consequently, start-stop systems are often configured such that the threshold brake pressure increases as the gradient of the surface on which the vehicle is traveling increases. One example of such a start-stop system is shown by Yu et al. in U.S. Pat. No. 8,998,774 B2. Therein, a vehicle is provided with a controller that is configured to shut down an engine in response to a brake effort exceeding a first threshold and to restart the engine in response to the brake effort decreasing below a second threshold. The first threshold and the second threshold are based on an estimated vehicle mass and a road gradient and are offset from each other to provide a hysteresis region.
However, the inventors herein have recognized potential issues with such systems. As one example, a fixed offset may not fully reduce inadvertent engine stops and restarts. For example, drivers often overestimate the amount of brake pressure for bringing the vehicle to a stop on an incline. As a result, drivers frequently trigger the start-stop system to inadvertently shut down the engine when stopping on an incline. Furthermore, when a driver brings the vehicle to a stop on an incline, the driver typically relaxes their foot and releases the brake pedal a greater amount than when stopping on a flat surface. This can lead to the engine being restarted before engine torque is needed. A start-stop system that is more intuitive for the driver to operate and reduces inadvertent shutting down and/or restarting of the engine when stopping a vehicle on an incline is therefore desirable.
In one example, the issues described above may be addressed by a method, comprising: stopping a vehicle engine when a brake value related to vehicle braking force exceeds a first threshold brake value related to a first function of road gradient; and restarting the engine when the brake value falls below a second threshold value related to a second function of road gradient, the difference between the first and second threshold brake values varying based on a magnitude of the road gradient. In this way, inadvertent engine shut downs and restarts may be reduced.
As one example, the first function of road gradient and/or the second function of road gradient may define a proportional relationship between the road gradient and the corresponding threshold brake value, such that as a magnitude of the road gradient increases, the corresponding threshold brake value increases. As another example, the first function of road gradient and/or the second function of road gradient may define a relationship between the road gradient and the corresponding threshold brake value in which the corresponding threshold brake value tends to a maximum value as the road gradient increases. In another example, the first and second functions of road gradient may be defined such that the difference between the first and second threshold brake values increases as the road gradient increases until a maximum value is reached. As still another example, the first and second functions of road gradient may be defined such that the difference between the first and second threshold brake values tends to a maximum value as the road gradient increases. At least one of the first and second threshold brake values may comprise a threshold brake pressure value, a threshold braking torque value, a brake pedal displacement value, or a threshold value based on any other parameter or characteristic of the brake system. The first and second threshold brake values may relate to the same parameter or characteristic of the brake system. Alternatively, the first and second threshold brake values may relate to different parameters or characteristics of the brake system. For example, the first threshold brake value may relate to a brake pressure, and the second threshold brake value may relate to a braking torque. By varying the first and second threshold brake values as well as the difference between the first and second threshold brake values based on the road gradient, a driver of the vehicle may experience increased, more intuitive control over engine operation, increasing driver satisfaction. By reducing inadvertent engine shut downs and restarts, fuel economy may be increased, vehicle emissions may be reduced, and vehicle wear may be decreased.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.